1. Field of the Invention
The present invention relates to an image forming apparatus and a process cartridge.
2. Description of the Related Art
In electrophotographic image forming apparatuses, an image is formed by subjecting a photoconductor used as an image carrier to a charging process, an exposure process, a developing process, and a transfer process.
Electrical discharge products produced in the charging process and non-transferred toner that is not transferred to a transfer material sometimes remain on the photoconductor. Therefore, a cleaning process is performed on the photoconductor after the transfer process to remove the electrical discharge products and remaining toner from the photoconductor.
As a cleaning system used in the cleaning process, a system using a rubber blade is generally well known. The rubber blade is inexpensive, simple in mechanism, and excellent in cleaning capability.
However, because the rubber blade is used to remove residual materials from the surface of the photoconductor by being pressed against it, there is large stress due to friction between the surface of the photoconductor and a cleaning blade as the rubber blade. Therefore, the rubber blade is worn, and the surface layer of the photoconductor or of an organic photoconductor in particular is worn, which causes both lives of the rubber blade and the organic photoconductor to be reduced.
Recently, small-sized toner particles are increasingly used for image formation to meet demands for high image quality.
In the image forming apparatus using the small-sized toner particles, residual toner particles as non-transfer toner quite often pass through under the cleaning blade. Particularly, when dimensional accuracy of the cleaning blade or the assembly accuracy are insufficient or when the cleaning blade partly vibrates, much more of the toner particles pass through under the cleaning blade, which prevents formation of high-quality images.
Therefore, to extend the life of the organic photoconductor and maintain high image quality over a long period, it is necessary to reduce degradation of a material due to friction and improve the cleaning capability.
One of general methods of reducing friction is a method of supplying a lubricant to the surface of the photoconductor, making the supplied lubricant uniform by the cleaning blade, and forming lubricant coating thereon.
When the lubricant is used, and if an applied amount of the lubricant is too little, then the lubricant is not much effective in wear and flaws of the image carrier or in degradation of the blade. If the lubricant is applied too much, then excessive lubricant is accumulated on the photoconductor to cause “flowing” of an image, or the excessive lubricant may be mixed into a developer to cause performance of the developer to decrease. Thus, it is necessary to define the supplied amount of lubricant.
A configuration of controlling the supplied amount of lubricant is proposed in Japanese Patent Application Laid-Open No. 2000-75752.
Japanese Patent Application Laid-Open No. 2000-75752 discloses that the charging system is contact charging, a tandem machine is provided by arranging imaging units in line that create individual images of a plurality of colors, and that the lubricant is applied at least 0.4 gram or more by the time when a driving distance of the photoconductor reaches 525 meters which corresponds to about 1250 sheets of paper of A3 in vertical orientation. This image forming apparatus is excellent in specific image formation based on such a condition that contents of an image to be formed i.e. toner consumption or the like is almost constant.
Recently, on the other hand, so-called alternating current (AC) charging tends to be used for the charging process. The AC charging is performed by using a charging roller or the like that is charged by superimposing an AC voltage on a direct current (DC) voltage.
The AC charging has excellent capabilities such as high uniformity of a charging potential on a photoconductor, less generation of oxidized gas such as ozone and NOx, and minimization of a device. On the other hand, the AC charging has disadvantages such that positive/negative electrical discharge is repeated hundreds to thousands times per second between a charging element and a photoconductor according to frequencies of a DC voltage to be applied, which causes degradation of the surface layer of the photoconductor due to a large number of electrical discharges, to be accelerated.
As a method of suppressing progress of the degradation on the surface layer of the photoconductor, a method of applying a lubricant such as zinc stearate to the surface layer and executing the charging process is proposed in Japanese Patent Application Laid-Open No. 2005-17469.
The content disclosed in Japanese Patent Application Laid-Open No. 2000-75752 shows that although it is effective in specific image formation in which an image forming condition such as toner consumption or the like is uniform, it is ineffective in formation of various types of images such that failure frequently occurs in an initial stage particularly when the photoconductor is started. Furthermore, even if the consumption is made to be constant, the property of the cleaning capability changes depending on each production lot of lubricants.
On the other hand, the following problems arise in the method of executing the charging process after the lubricant is applied, as disclosed in Japanese Patent Application Laid-Open No. 2005-17469.
When the lubricant is applied on the photoconductor, the energy of the AC charging is first adsorbed by the lubricant and thus the energy is difficult to reach the surface of the photoconductor, which allows protection of the surface thereof by suppressing the degradation due to AC charging.
However, if AC charging is performed in such a manner that the AC voltage is superimposed on the DC voltage based on such a configuration that a charging roller is provided close to the photoconductor, the coating of the lubricant formed on the surface of the photoconductor disappears by being applied with the AC charging.
A disappearing speed is extremely fast as compared with that of corona discharging, and thus how to form lubricant coating is largely different from that of the corona discharging.
If a unit of applying the lubricant while AC charging is continued and image formation is also continued is used, the phenomenon in which degradation of the surface of the photoconductor is progressing caused by the AC charging occurs more quickly than the effect that the lubricant is applied to the surface thereof and the coating is formed to thereby protect the photoconductor, depending on an applied amount of lubricant.
If the applied amount of lubricant is increased to avoid this problem, some problems such as blurring or a change in property of a developer arises. On the other hand, if the applied amount of lubricant is suppressed and a sufficient amount of lubricant is not thereby uniformly applied over the surface thereof, then the degradation of the surface thereof is accelerated by being applied with AC charging.
Although Japanese Patent Application Laid-Open No. 2005-17469 discloses a technology on protection of the surface of the photoconductor with the lubricant, it does not teach or suggest influences on an image and further on the cleaning capability due to behavior of the lubricant upon charging after the lubricant is applied as explained above. Therefore, the problem on how the lubricant affects the cleaning capability including disappearance of the lubricant remains unsolved.
As disclosed in Japanese Patent Application Laid-Open No. 2005-17469, the lubricant, zinc stearate in particular, applied to the surface of the photoconductor sometimes exists in a form of powder or mass. In this case, the surface of the photoconductor becomes nonuniform depending on whether the lubricant is deposited. Therefore, when the deposited lubricant being the powder or mass as it is passes through the charging process, the coat on a certain portion of the surface where the lubricant is deposited is not scraped. However, a portion on the surface where no lubricant is deposited is not protected with the lubricant, which causes the coat of the surface layer to be scraped. Based on this situation, image formation is performed under actual use conditions, defects such as streaks appear on the image due to irregular wear affected by toner input and an image area or a non-image area.
Furthermore, if powder or mass of the lubricant is present on the photoconductor, the powder or the mass flies or moves onto the charging roller when passing through the charging roller, and melts thereon. The melted powder or the mass is solidified with toner components such as external additives of toner. The resistance becomes high at portions where the lubricant has locally melted and solidified, which may cause uneven charging.
A detailed study is conducted on the state condition of zinc stearate in an initial state or a state after time passes when the AC voltage is applied using a proximity charging method. There is sometime a case where a portion on the surface of the photoconductor is unevenly applied with the lubricant upon start of the photoconductor. As for the portion with the lubricant deposited thereon, when the portion where the lubricant has once been formed is subjected to AC charging, the coating of the lubricant disappears, but the coating is formed again by again applying the lubricant to the surface thereof.
On the other hand, according to experiments conducted by the inventors of the present invention, it is found that if the portion of the surface where no lubricant is deposited due to uneven charging is once degraded by being subjected to AC charging, and even if the lubricant is applied to the degraded portion, the lubricant is difficult to be kept deposited thereon.
Consequently, in the position where AC charging is applied in the initial stage before the coating is formed, the lubricant cannot be retained and degradation of the portion is progressing with time, while in the portion where the coating of the lubricant is formed in the initial stage, the coating is newly formed by applying the lubricant and thus the degradation does not easily progress. It is, therefore, clear that if the lubricant is unevenly applied in the initial stage, this causes local degradation to progress.
According to the result, the method disclosed in Japanese Patent Application Laid-Open No. 2005-17469 may cause lubricant retention not to be ensured when the lubricant is unevenly applied or the time passes. It is confirmed through the experiments that this case is caused to degrade the photoconductor or not to keep high-quality images.
When the zinc stearate is used as the lubricant, to apply this material, a following method may sometimes be employed. The method is such that a brush is pressed against a bar of zinc stearate to make powder of zinc stearate, the powder thereof is made to drop on the photoconductor and be deposited thereon, and the powder thereof is crushed and spread out by a blade or the like.
To increase the amount of the zinc stearate on the photoconductor, it is generally thought of that the particle size of the zinc stearate to be applied to the photoconductor is increased or the number of particles is increased, and in many cases the force to press the brush against the bar of the zinc stearate is increased. However, when the AC charging by the charging roller is used in the charging process, as explained above, the following problem tends to arise. The problem is such that various substances are deposited on the charging roller and further these substances are firmly fixed thereto, which causes resistance of the charging roller to be locally increased, and defective charging is caused to occur in this local portion.
The powder of the zinc stearate generated by pressing the brush against the bar of the zinc stearate moves to the developer, in addition to movement of the powder to the charging roller. The chargeability of the developer thereby changes, which may cause failure in density reduction. As explained above, when the zinc stearate is applied to the photoconductor, because the above mentioned process is used, it is not possible to avoid that the powder of the zinc stearate moves to some places other than the photoconductor.
Because the lubricant existing on the photoconductor in mass or powder form moves to some places from the photoconductor, the following method can be used such that the lubricant applied to the photoconductor is present thereon in an or film form but not in mass or powder. Incidentally, the term “irregular” as used herein refers to a state that cannot be described as a specific form, i.e., a form that cannot be explained by indices such as a particle diameter or a degree of circularity.
The method is such that the lubricant is supplied to the photoconductor in the form of particles or mass, and is sufficiently spread out using a blade or so. However, there is a limit to spread out the lubricant supplied in the form of particles or mass and there is a slight space between the blade and the photoconductor, and therefore, the powder passes through the space. If the blade is pressed more strongly against the photoconductor to prevent passage of the powder of the lubricant, then wear of the photoconductor is accelerated. As explained above, if the lubricant is supplied to the photoconductor in the form of mass or particles, the mass or the particles cannot perfectly be removed.
When the powder of bar of the lubricant is supplied to the photoconductor by using the brush, there is no particular problem in the short term. However, when it is used over the long term, the powder of the lubricant on the photoconductor moves to some places other than the photoconductor. The movement causes change in property of the developer or defective charging of the charging roller to occur, and thus high-quality image formation cannot be maintained over the long term.
Japanese Patent Kokoku Publication No. S51-22380 and Japanese Patent Application Laid-Open No. 2004-333961 have proposed a technology of applying solid lubricant containing zinc stearate as a main component to the surface of the photoconductor and forming lubricant coating on the surface thereof to extend the lives of the photoconductor and the cleaning blade.
In the conventional technology, when zinc stearate is used as a solid lubricant, a protective-agent bar made of zinc stearate is pressed by a brush and the zinc stearate is shifted to the brush, and the zinc stearate is supplied from the brush to the surface of the photoconductor. Because the zinc stearate is comparatively hard, when the brush is pressed against it, the zinc stearate is made powder and the powder is deposited on the surface thereof. By spreading the powder using the blade or the like, the zinc stearate is formed on the photoconductor in film form.
However, part of the powdery zinc stearate deposited on the photoconductor keeps its powdery form even when it passes through under the cleaning blade, and thus, the powdery zinc stearate is easily deposited on a charger in the charging process. Particularly, when the charging roller is used for the charger, the photoconductor and the charging roller are in contact with each other or have only a distance of hundreds micrometers or less between the two. Therefore, there is extremely high probability in which particles of zinc stearate are deposited on the charging roller. These methods have further disadvantages such that when the DC voltage is superimposed on the AC voltage to be charged to the charging roller, the zinc stearate deposited on the charging roller melts thereon by charging energy, the melted material is firmly fixed to the charging roller in film form while involving toner components remaining on the surface of the photoconductor after being cleaned, which causes the resistance in the portion with melted material fixed thereon to increase, and thus uneven charging easily occurs. Therefore, a lubricant (protective agent) that does not become easily a fine powder even if the brush is pressed against it is required.
When a protective-agent bar formed of a bar-type protective agent is pressed by the brush to supply the protective agent to the photoconductor, the following measure is found effective to cause the protective-agent bar to hardly become the fine powder. The effective measure is such that a soft material is used for the protective agent and the protective-agent bar is not made powder caused by impact occurring when the brush is pressed against the protective-agent bar, but the protective agent is deposited on the end of the brush, and when the end of the brush with the protective agent deposited thereon is brought into contact with the photoconductor, the protective agent shifts from the brush to the photoconductor.
However, in an initial stage in which the protective agent is to be deposited, the protective agent is not deposited on the brush, and thus the protective agent is difficult to be supplied to the photoconductor. If image formation is repeated when the protective agent is not on the photoconductor, then a portion without the protective agent is oxidized and degraded due to energy of charging. The oxidation and degradation cause the cleaning capability of residual toner on the photoconductor to be decreased, the edge of the cleaning blade to be worn, and so-called filming that toner components are deposited in film form to easily occur. If these phenomena once occur, defects are hardly resolved even if the protective agent can be supplied again, which results in replacement of the photoconductor or the process cartridge.